Solid electrolytic capacitors, such as tantalum capacitors, are traditionally known for their high capacitance value and compactness. Fuses are often employed in such capacitors to prevent burning upon exposure to overcurrent conditions. A typical fuse assembly employs a small conductive wire that disintegrates in response to excessive electrical current. Typically, one end of the fuse is soldered to a metal conducting foil edge of an electrolytic capacitor element and the other end of the fuse is soldered to a metal collector bus. When an electrolytic capacitor element fails, it creates a short circuit through which energy stored therein and in the circuit may discharge. The fuse disintegrates in response to the excessive current resulting from this discharge, which breaks the electrical connection between the failed element and the collector bus.
One problem with such conventional fused capacitors, however, is that they can no longer function in the desired manner. In response to these problems, attempts were made to develop capacitors that employed resettable fuses. U.S. Pat. No. 6,882,520 to Kamigawa, et al., for instance, describes solid electrolytic capacitors that contain a current control layer prepared from an insulating polymer (e.g., polyethylene) having electrically conductive particles (e.g., carbon black) admixed therewith. Such materials are often referred to as polymer positive temperature coefficient (“PPTC”) fuses. At a temperature associated with a current overload, a PPTC fuse is designed to expand and break the conductive pathways between the conductive particles. Upon cooling, the fuse may contract to close the circuit, thereby rendering the fuse at least partially “resettable.” Unfortunately, resettable fuse capacitors, such as described in Kamigawa, et al., are still not completely satisfactory for use in many commercial applications. Without intending to be limited by theory, the present inventors believe that one of the problems with such capacitors is that the resin used to encapsulate the electrolytic capacitor limits the expansion of the resettable fuse to such an extent that it does not function to its full capacity. Further, the thermal stresses induced by expansion of the fuse may also lead to the formation of defects in the encapsulation resin.
As such, a need currently exists for an improved electrolytic capacitor assembly that includes a resettable fuse.